cell responses are elicited to some but not all possible targets of the immune response, and the rules that govern epitope selection are not completely clear. The main goals of this grant application are first to investigate epitope selection in the human T cell response to pathogens and second to investigate mechanisms of pathogens that affect epitope selection. Vaccinia virus and human herpesvirus 6 will be used as model pathogens. These are large-genome DMA viruses with many potential epitopes for which questions of epitope selection and immune hierarchy are particularly relevant. The proposal has three specific aims. Aim 1 is to determine factors that control epitope selection of the human CD4+ T cell response by using vaccinia virus as a model. Several potential factors affecting epitope selection will be evaluated, including peptide-MHC dissociation kinetics, antigen expression level and location in the infected cell and/or viral particle, and whether the antigen is processed using conventional endosomal processing or autophagy pathways. Aim 2 is to characterize the human CD4+ and CD8+ T cell response to HHV-6 virus. HHV-6 is a relatively recently discovered beta herpesvirus that establishes a long-lasting latent infection, and most people are carriers as a results of childhood exposure. Viral reactivation can occur under immunosuppressive conditions such a post-transplantation therapy or AIDS. Despite the central role of cellular immunity in controlling this virus, very little is known about the T cell response. In work directed at Aim 2, we will characterized the T cell response to HHV-6, study its specificity, and determine if factors important in epitope selection in vaccinia virus also apply to HHV-6. Many viruses have mechanisms to evade or modulate the immune response against them. AIM 3 is to characterize modulation of antigen processing and presentation pathways in vaccinia-infected cells, and to determine whether HHV-6 also has mechanisms to alter antigen processing pathways in infected cells.